1. Field of the Invention
The invention relates to bone fixation; and, more particularly, to apparatus and method for repairing bone fractures.
2. Related Art
Various surgical plates have been employed in the treatment of traumas to facial or cranial bone structure, plastic surgery, reconstructive facial surgery, and the like to hold the bone sections or fragments in place during the healing process. The surgical plates are positioned against the surface of the bone sections which must be held together and fixed to the bone by mechanical fasteners such as bone screws, wire sutures, or other fasteners which secure the plate to the bone surface. The fasteners are securely pressed or embedded in the bone to prevent the surgical plates from being pulled away from the surface of the bone sections. U.S. Pat. Nos. 4,966,599 and 5,290,281 disclose examples of bone stabilization plates which are secured to the bone structure, for example, facial or cranial bones, by bone screws. U.S. Pat. No. 4,655,203 discloses a surgical device for immobilization of bone fracture which include a stiff plate and stiff fixing elements which may be pressed into notches formed in the bone.
Thus, present methods of bone fixation are:
Bone plates PA1 Wire tension bands PA1 Screws PA1 Intramedullary rods PA1 Cerclages (tie wraps; nylon, metal, or wire) PA1 External fixation (pins and frame)
All of these prior art methods are inherently distinct and each seems to be the method of choice for certain fracture cases. There is also considerable agreement on the indications and contraindications as to when each is used.
The bone plate attains compression of the fragments through a wedge geometry of the slot in which the screw hole rides. As the screw is tightened, the wedge geometry of the slot forces the plate to the right while pulling the bone fragments into position and compression. The amount of compression depends on many variables; access to the surgical site, the fit of the bone to the plate and how accurately the screws are placed in the holes. The distribution of the compressive forces are located around the plate. Also the plate is most likely to have six to eight screws.
This surgical procedure requires some skill; the plate must be contoured to the bone and the holes must be drilled accurately to get proper reduction and compression.
An undesirable consequence of the bone plate is the fact that, after a short time, it changes the stress flow in the bone by becoming a stress shield which weakens the bone. As a result it is usually removed. The bone plate may also produce a pressure necrosis on the bone and periosteum. They promote the formation of adhesions between the gliding planes and can produce a bursa-like irritation. Consequently, these can interfere with rehabilitation, and they should be removed when they are no longer of benefit. See Uhthoff, H. K., and Dubc, F. L. (1971): "Bone structure changes in the dog under rigid internal fixation" Clinical Orthrop. Rel. Res., 81:165-170 and Uhthoff, H. K., and Finnegan, M. (1983): "The effects of metal plates on post traumatic remodeling and bone mass" J. Bone Joint Surg., 1983 65-B:66-71.
Wire used to hold bone fragments together is usually passed through drilled holes, looped and tied. It is not practical at all for the metal wire to put compression into the bone. For it to do so the wire must be stretched like an elastic. But this is an impractical application for a wire because its elongation is such a tiny percentage of the length. Also, because of the relative strengths of the wire to bone, the bone will always fail in bearing before the metal wire will stretch. To use wire as a tension band means that the wire can hold the bone fragments in approximate location and can take out tension loads but it cannot apply an active form of compression into the fragments.
Screws (lag screw) used by themselves work only in special case where the break is overlapping sufficiently for the screw to go through both walls of the bone. Screws add compression in that direction only (traverse) and not axially where it can be the most beneficial. Screws can be used in comminuted fractures but only under specific conditions.
Cerclage bands that wrap around the bone again work only in ideal cases where the break provides sufficient overlap. Like screws, they work only in circumferential compression (no axial compression).
Intramedullary rods are used primarily in the long bones of the body, such as the femur. They are inserted into the femur above and just inside the greater trochanter and down the intramedullary canal. At present, there is an increasing agreement that a procedure known as Kuntscher's intramedullary nail, supplemented with interlocking design, is the treatment of choice for essentially all closed fractures of the femur located between the lesser trochanter and femoral condyles, regardless of the fracture pattern or degree of comminution.
External fixation consists of large diameter wire rods that go through the limb and bone completely to an external frame. Usually there are 4, 6, or 8 wires through the limb to hold the fractured bones in position. External fixation is used primarily for serious fractures, most likely comminuted, where access to the fracture site is compromised, and/or orientation of fragments difficult. They are most commonly used with severe open fractures and considerable soft tissue damage. The patient must deal with a very inconvenient and bulky device for a long time. There is also a risk for infection over the length of time it is installed.
There is thus a need for a process for holding bone fragments in tight approximation while allowing a small amount of load variability which encourages bone regeneration.